I know it's to follow the same api as for Velocity (nhibernate.caches.velocity) but wanted to know if anyone already had some production experience of using it and if they knew of any particular tips or problems?

I haven't used AppFabric as second level cache but I've used NCache as second level cache for Nhibernate and results were quite satisfactory.By default NHibernation provides a stand alone InProc (in process) first-level (L1) cache, or session-level cache. However in addition, , NHibernate also supports a second-level cache to be plugged in to exploit the benefits of a sophisticated enterprise-grade caching infrastructure.